react-datetime renders too much html elements without a reason

### I'm Submitting a ...

```
[ ] Bug report
[ ] Feature request
[x] Support request
```

### Steps to Reproduce

If the datepicker is loaded into the DOM, it already exists out of more than 70 DOM-nodes. Therefor if a lot of datepickers are rendered on screen, the UI becomes very laggy.
### Expected Results

These DOM-nodes should only be rendered when the datepicker is open.
### Actual Results

Only render the datepicker when it is open.

### Other Information (e.g. stacktraces, related issues, suggestions how to fix)

/

Start by reproducing the issue with multiple datepickers and inspect the datepicker rendering path described in the report. Done means a closed datepicker does not render its calendar elements into the DOM, while opening it still works and multiple datepickers no longer make the UI laggy.

Written by the indexing model from the issue text.